A high-speed bucket belt conveyor usually comprises a number of metal buckets attached at intervals of approximately 160 mm. to an endless belt, by fixing bolts passed therethrough. Conventionally, the belt moves vertically at a speed of between approximately 200 and 300 m. per minute. Therefore, a slip of the belt, due to overloading or for other reasons, causes great friction between the bucket fastening bolt and a belt driving wheel, which can produce a spark that may set afire the dust arising from the grain, etc., being conveyed and thereby cause a dust explosion.
To prevent disasters of this kind, measures have been taken to determine the belt speed by detecting the passage of a bucket with a contactless switch and, thereby, detect the belt slip.
With the buckets usually spaced very closely, however, oscillating and magnetic contactless switches are not suited for such bucket detection. Meanwhile, small contactless switches featuring a high detection speed must be installed as close as possible to the object to be detected because of their generally limited detecting distance. But, in actuality, extensive swinging of the belt does not permit such maximal close installation. Large contactless switches, on the other hand, have great enough detecting distance to stand well clear of the swinging belt. But they are so sensitive that they detect the next bucket before the detection of a preceding bucket has been completed.
An object of this invention is to provide a speed drop detector for a high-speed bucket belt conveyor that is capable of surely detecting rapidly moving, closely spaced buckets, setting a speed drop rate with high accuracy irrespective of the bucket speed, and working in conformity with various speed drop rates.
Another object of this invention is to provide a speed drop detector for a high-speed bucket belt conveyor that is capable of surely detecting a speed drop even if buckets are spaced farther away from each other where belt ends are joined together than in other parts of the belt.
Still another object of this invention is to provide a speed drop detector for a high-speed bucket belt conveyor that is versatile and can be used in accordance with the operating conditions of the belt conveyor.